Recent studies have revealed that overexpression of multidrug resistance (mdr) gene in some human neoplasms is correlated with poor prognostic responses of these tumors to chemotherapeutic agents. Elucidating the molecular basis of mdr gene expression in normal and in malignant cells may provide important insights into clinical applications of chemotherapy of these tumors. We propose here to investigate the regulation of mouse mdr gene expression in hepatoma-derived cells and in uterine epithelial (UE) cells of pregnant animals. In both cases, mdrl gene is activated. Preliminary results demonstrated that mdrl gene expression can be activated by steroid hormone in hepatoma cells. It is our aim to determine whether the activation mechanism is at the transcriptional or at the posttranscriptional levels and to elucidate the possible hormone responsible DNA elements in the mdrl locus. Furthermore, a 33-bp sequence located in the promoter region of mdrl gene is involved with mdrl expression in hepatoma cells. We propose experiments to further characterize this cis-acting DNA element. Possible trans-activating factor(s) interacting with this DNA element will be purified and genes encoding these factors will be cloned. The activation mechanism of mdrl gene in UE cells during pregnancy will be studied using primary culture derived from UE. This primary culture maintains its apparent in vivo characteristics. The role of steroid hormones in this activation mechanism will be studied; and possible cis-acting DNA sequences in the mdr gene will be determined by transient transfection using microinjection method. These proposed studies may help us to learn the mechanisms of mdr gene regulation in normal and malignant cells.